KR102250330B1 - Method for resistance spot welding of steel sheet - Google Patents

Abstract

Disclosed is a method for spot welding a steel sheet. In one embodiment, the spot welding method of the steel sheet comprises steps of: pressing a pair of electrodes in contact with the upper and lower portions of a weld target portion of the steel sheet to be joined; applying and maintaining a first energization current value (I1) to the weld target portion while maintaining the pressure of the electrode to conduct primary energization; and performing secondary energization by increasing and maintaining the current with a slope from the first energization current value (I1) to a second energization current value (I2) while maintaining the pressurization.

Description

Resistance spot welding method of steel sheet {METHOD FOR RESISTANCE SPOT WELDING OF STEEL SHEET}

The present invention relates to a method for resistance spot welding of a steel sheet. More specifically, it relates to a resistance spot welding method of a steel sheet having excellent weld quality.

When assembling automobile parts, bonding and welding are essentially used. Among them, resistance spot welding uses upper and lower electrodes to fix and pressurize an object to be joined, such as a plate, and to conduct electric current between the electrodes while maintaining the pressure, and the electrical resistance heat generated inside the object to be joined. It is a welding method in which the surface to be joined is melted by using and at the same time, a joint is formed by pressure. Such resistance spot welding is most widely applied as it is suitable for vehicle body welding. During resistance spot welding, the quality of the welded part is very important in managing the quality (stiffness and durability characteristics, etc.) of an automobile body.

On the other hand, when the object to be joined using resistance spot welding starts to generate heat from the joint interface due to the contact resistance, when the object to be joined is an ultra-high strength steel, due to the high resistance and high stiffness characteristics, the electric resistance heating rate at the joint interface is very high. The high melting nugget is generated first, so that rapid growth occurs, and the possibility of occurrence of an intermediate expulsion is increased, leading to a decrease in welding quality.

In addition, the possibility of interfacial fracture that separates the bonded surface due to brittleness and high stiffness increases according to the amount of high alloy, and energy absorption capacity decreases when the bonded automobile parts collide, and the quality may be poor, leading to quality failure. have. To solve this problem, a method of welding by increasing the pressing force on the object to be joined is used, but it was impossible due to the limitations of mass production facilities. Therefore, it is necessary to improve the welding quality in order to secure the rigidity, durability and collision stability of the joint part of the automobile body.

Background technology related to the present invention is disclosed in Korean Patent Publication No. 10-1482390 (announcement on January 21, 2015, title of the invention: resistance welding apparatus and method).

According to an embodiment of the present invention, it is to provide a resistance spot welding method of a steel sheet having excellent welding quality by minimizing defects in welds.

According to an embodiment of the present invention, it is to provide a resistance spot welding method of a steel sheet excellent in the mechanical strength and impact performance improvement effect of the weld.

According to an embodiment of the present invention, it is possible to weld in a wide range of welding current to provide a resistance spot welding method of a steel sheet, which facilitates welding quality control.

According to an embodiment of the present invention, it is to provide a resistance spot welding method of a steel sheet capable of improving welding quality by preventing the occurrence of flying when welding high alloy and ultra high strength steel parts.

One aspect of the present invention relates to a method of resistance spot welding of a steel sheet. In one embodiment, the resistance spot welding method of the steel sheet comprises the steps of contacting and pressing a pair of electrodes on the upper and lower portions of the welding target portion of the steel sheet to be joined; _{Applying and maintaining a first current value (I 1} ) to the portion to be welded while maintaining the pressure of the electrode to perform primary current; And performing secondary current by increasing and maintaining the _{current from the first} current value (I 1) to the second current value (I _{2) while maintaining the pressurization.}

In one embodiment, the bonded steel sheet may include at least one of ultra-high strength steel and hot forming steel sheet having a tensile strength of 540 MPa or more.

In one embodiment, the holding time of the first conducting current value (T ₁ ) and the holding time of the second conducting current value (T ₂ ) may satisfy the relationship of Equation 1:

[Equation 1]

T ₂ ≤ T ₁ ≤ 3*T ₂ (unit: ms).

In one embodiment, the first conduction current value (I ₁ ) may satisfy the relationship of Equation 2:

[Equation 2]

3.0kA ≤ I ₁ ≤ Maximum current (kA) of non-flying of the bonded steel sheet.

In one embodiment, when the current increases from the first conduction current value to the second conduction current value, it may be increased for a time of 50% or less of _{the second conduction current value holding time T 2.}

In one embodiment, prior to performing the first energization, applying and maintaining a preliminary current value to the portion to be welded to perform preliminary energization may be further included.

In one embodiment, the preliminary conduction current value Ip may be a current value _{equal to or greater than the first conduction current value I 1.}

When applying the resistance spot welding method of the steel plate of the present invention, the welding quality is excellent by minimizing the defect of the welding part, the mechanical strength of the welding part and the effect of improving the collision performance are excellent, and even when the ultra-high strength steel is applied, welding is possible in a wide range of welding currents. Welding quality control is easy, and welding quality can be improved by preventing the occurrence of flying during welding of high alloy steel and ultra high strength steel parts with high alloy content.

1 shows a resistance spot welding method of a steel sheet according to an embodiment of the present invention.
2 shows a resistance spot welding current pattern according to an embodiment of the present invention.
3 shows a conventional resistance spot welding current pattern.
4 is a graph comparing the range of welding current during resistance spot welding of a general steel plate and an ultra-high strength steel.
5 is a photograph showing a welding defect of ultra-high strength steel.
6(a) shows the scattering of the welded part of the hot-formed part to which the conventional resistance spot welding is applied, and FIG. 6(b) is a photograph showing the formation of pores due to the scattering of the welded part during the conventional resistance spot welding.
7(a) is a cross-sectional view of the welding part after the first current is applied in Comparative Example 2, and FIG. 7(b) is a cross-section of the welding part at the time when the current is applied up to the second current value during the second current of the embodiment. It's a picture.
Figure 8 (a) is a comparative example 1 shows the welding nuggets, Figure 8 (b) is a photograph showing the example welding nuggets.
9 is a photograph showing a welding defect of Comparative Example 2.
10 is a graph showing the size of the nugget diameter according to the current range applied to the secondary energization during resistance spot welding of Examples and Comparative Example 2 of the present invention.

Hereinafter, the present invention will be described in detail. In this case, when it is determined that a detailed description of known technologies or configurations related to the present invention may unnecessarily obscure the subject matter of the present invention, a detailed description thereof will be omitted.

In addition, terms to be described later are terms defined in consideration of functions in the present invention and may vary according to the intention or custom of users or operators, so the definitions should be made based on the contents throughout the present specification describing the present invention.

Resistance spot welding method of steel plate

One aspect of the present invention relates to a method of resistance spot welding of a steel sheet. 1 shows a resistance spot welding method of a steel sheet according to an embodiment of the present invention.

Referring to Figure 1, the resistance spot welding method of the steel sheet (S10) electrode contact and pressing step; (S20) the first energization step; And (S30) a second energization step. More specifically, the resistance spot welding method of the steel sheet includes the steps of (S10) contacting and pressing a pair of electrodes on the upper and lower portions of the welded steel sheet; _{(S20) applying and maintaining a first electric current value (I 1} ) to the welding target portion while maintaining the pressure of the electrode to perform primary electric current; And (S30) performing secondary energization by increasing and maintaining the current with a slope _{from the first} current value I 1 to the second current value I _{2 while maintaining the pressurization.} .

Hereinafter, the resistance spot welding method of the steel sheet of the present invention will be described in detail step by step.

(S10) Electrode contact and pressurization step

The step is a step of contacting and pressing a pair of electrodes facing each other on the upper and lower portions of a portion to be welded of the steel sheet to be joined. In one embodiment, the bonded steel sheet may include at least one of ultra-high strength steel and hot forming steel sheet having a tensile strength of 540 MPa or more. For example, the steel sheet for hot forming having a tensile strength of 540 MPa or more may include a plated steel sheet including an aluminum (Al) and zinc (Zn)-based plating layer, an unplated steel sheet, and the like. For another example, it may include a third-generation steel sheet having a high alloying amount. The third-generation steel sheet may include QP (Quenching and Partitioning), medium Mn steel, and TRIP-based steel.

In the present invention, as the tip diameter of the electrode increases, the maximum growth width of the size of the nugget increases, so that the effect can be further maximized. In one embodiment, the electrode may use an electrode tip having a diameter of the electrode tip of Φ8mm or more. In the above conditions, the size of the nugget size may be further increased under the condition of maximum heat input at a high current, so that the nugget diameter growth effect may be excellent.

(S20) 1st energization step

The step is a step of applying and maintaining the first _{current value (I 1} ) to the portion to be welded while maintaining the pressure of the electrode to perform primary current. The primary energization of the present invention is different from the energization of a high current/short time spike type, which is performed for the purpose of removing the conventional plating layer or stabilizing the surface of the steel sheet to be joined. , By applying a current value at a level that does not cause flying for a long time (medium cuurent/long time), the welding heat affected zone (HAZ) area and corona bond (corona bond) formed around the nugget by welding heat before the secondary energization. It is possible to prevent scattering of the melt by expanding the area as a ring-shaped part, which is not actually melted but is pressed into a solid state by pressure at a high temperature).

2 shows a resistance spot welding current pattern according to an embodiment of the present invention. Referring to FIG. 2, the holding time T _{1 of} the first conducting current value and the holding time T ₂ of the second conducting current value may satisfy the relationship of Equation 1 below:

[Equation 1]

T ₂ ≤ T ₁ ≤ 3*T ₂ (unit: ms).

By applying the energization time, during the first energization, the welding heat affected zone (HAZ) and the corona bond region due to welding heat may be enlarged as much as possible, thereby preventing scattering of the melt.

In one embodiment, the first conduction current value (I ₁ ) may satisfy the relationship of Equation 2:

[Equation 2]

3.0kA ≤ I ₁ ≤ Maximum current (kA) of non-flying of the bonded steel sheet.

Under the above conditions, the welding heat-affected zone (HAZ) and the corona bond area of the welding target area can be enlarged as much as possible to prevent scattering of the melt, and the mechanical strength and welding quality of the welding portion can be excellent during the secondary energization described later . For example, the first conduction current value I ₁ may be 3 to 10 kA.

Preliminary energization stage

In another embodiment of the present invention, a step of applying and maintaining a preliminary current value to the region to be welded before performing the primary current after contacting and pressing the electrode to perform preliminary power. I can. Referring to FIG. 2, the preliminary conduction current value Ip may be a current value _{equal to or greater than the first conduction current value I 1 in one embodiment.} For example, it may be a current value equal to or greater than the first conduction current value _{I 1 and less than the second conduction current value I 2.}

In one embodiment, the preliminary conduction current application time may be less than _{the holding time (T 1) of the first conduction current value.} In the above conditions, even when the size of the first conducting current value is increased, fluttering does not occur, and welding quality may be excellent. For example, it may be 1 to 100 ms.

(S30) 2nd energization step

The step is a step of performing secondary energization by increasing and maintaining the current with a slope _{from the first} current value (I 1) to the second current value (I ₂ ) while maintaining the pressurization as shown in FIG. 2. to be.

When the current is increased with a slope as described above, it is possible to enter a stable high current to perform secondary energization, and to prevent sudden scattering of the nugget in the form of a melt formed through the first energization step. . In the case of performing secondary energization by increasing the current without having the slope, the molten material may scatter and the welding quality may deteriorate.

The secondary energization prevents scattering of the melt even when a high current is applied for a long time due to the current pattern having a slope and increased after the preliminary energization, the first energization, and the first energization, and the melting quality is excellent.

For example, the second conduction current value I ₂ may be 7.0kA to 15.0kA. In the case of performing the secondary energization under the above conditions, it is possible to prevent a phenomenon in which the welding quality is deteriorated due to scattering of the melt.

In one embodiment, the current increase time (Ts) for increasing the current from the first conduction current value to the second conduction current value may be 50% or less of _{the second conduction current value retention time (T 2 ).} When the current is increased under the above time conditions, the scattering of the melt can be prevented and the welding quality can be excellent. For example, the current increase time Ts may be 10 to 250 ms.

In one embodiment, the holding time (T ₂ ) of the second conducting current value may be 100 to 500 ms. Stable growth of the nugget size is possible under the above conditions, and the welding quality may be excellent.

On the other hand, Figure 3 shows a resistance spot welding conduction pattern conventionally practiced. 3(a) to 3(c), when welding by energizing under a conventional current waveform condition, the available welding current range is narrow due to expulsion before sufficient nugget growth is achieved in the low current region. It was difficult to ensure quality and uniform quality control was difficult.

On the other hand, a weld metal that is melt-solidified by welding and generated in a portion to be joined is referred to as a nugget. The size of the nugget grows in proportion to the total amount of heat input, and it is known as an important factor that influences the quality of the weld. Meanwhile, the welding current range generally refers to a current range less than the current value (lower limit current value) to which the nugget size formed in the bonding object is secured to a certain level to the occurrence of flying (upper limit current value).

4 is a graph comparing the range of welding current during resistance spot welding of a general steel plate and an ultra-high strength steel. Referring to FIG. 4, in the case of ultra-high strength steel, third generation steel sheet, and hot stamping steel, high current welding was not possible due to a flying phenomenon even at low current, unlike general steel, and the applied welding current range was narrow as well as welding quality. There was a limit to increase.

Figure 5 is a photograph showing the welding defects of the ultra-high strength steel, Figure 6 (a) shows the scattering of the welded part of the hot-formed part to which the conventional resistance spot welding is applied, and Figure 6 (b) is due to the scattering of the welding part during the conventional resistance spot welding. This is a picture showing pore formation. 5 and 6, in the case of ultra-high-strength steel, molding is difficult and springback is excessive, resulting in dimensional deviation due to poor molding dimensional accuracy, and easy flying during welding due to defective assembly surface plate matching (gap, tilting, etc.) Therefore, it is adversely affected in securing welding quality. As such, ultra-high-strength steel rapidly generates excessive resistance heating even at low current due to the increase in electrical resistance due to the addition of high alloying amount, high stiffness, and dimensional accuracy dispersion, and local non-uniform heat generation is concentrated, forming pores inside the nugget. The weldability is very inferior because stable nugget growth is difficult, but the prior art has limitations in securing welding quality.

On the other hand, in a situation where the application rate of ultra-high-strength steel is increasing in order to secure the weight and stability of the vehicle body in recent years, the strength and quality of the weld should also be increased, but the welding characteristics of the ultra-high-strength steel have been in reverse. In particular, the weldability of the hot stamped steel sheet for hot forming and the third generation steel sheet was more vulnerable. In the case of such high-strength steel, the contact resistance and volume resistance are high due to the high stiffness and alloy amount, so the heating rate is high when the welding current is applied, so it is sensitive to the occurrence of intermediate expulsion, and the nugget growth is limited due to the occurrence of intermediate expulsion. There was a difficulty in securing quality due to the occurrence of internal defects such as.

In particular, in the case of Al, Zn-based plating hot stamping parts, the interdiffusion layer and IMC (intermetallic compound) are formed due to the formation of an oxide film and alloying reaction in the heat treatment process in the furnace, which increases the surface resistance, resulting in increased sensitivity to the occurrence of flying. It is very high, and the weldability is more vulnerable.

On the other hand, when applying the resistance spot welding method of a steel plate according to the present invention, the welding quality is excellent by minimizing the defect in the welding part, the mechanical strength of the welding part and the impact performance improvement effect are excellent, and welding is possible in a wide range of welding current, so that welding quality management It is easy to use, and it is possible to improve welding quality by preventing the occurrence of flying when welding high alloy steel and ultra high strength steel parts with high alloy addition amount.

Hereinafter, the configuration and operation of the present invention will be described in more detail through preferred embodiments of the present invention. However, this has been presented as a preferred example of the present invention and cannot be construed as limiting the present invention in any sense.

Example

Two sheets of ultra-high-strength steel with a tensile strength of 540 MPa or more (a hot-formed steel sheet with a thickness of 1.4t for hot stamping formed with an Al-Si plated layer) as a bonded steel sheet, and at the top and bottom of the area where the ultra-high-strength steel abuts (the part to be welded). A pair of electrodes (tip diameter Φ8mm) were respectively brought into contact and pressurized. While maintaining the pressurization, a pre-conduction current value of 6 kA or more was applied and maintained for 1 to 100 ms to the welding target portion, and pre-energization was performed. Then, while maintaining the pressure of the electrode, a first current value (I ₁ ) of 6.0 kA was applied to the portion to be welded and maintained for 400 ms to perform primary energization. While maintaining the pressurization, the _{current has a slope from the first} current value (I 1) to the second current value (I ₂ ) (increases to 9.5 kA for a time of 50 ms) and increases and maintains the second current. And resistance spot welding.

During the resistance spot welding, the holding time (T ₁ ) of the first conducting current value and the holding time (T ₂ ) of the second conducting current value satisfied Equation 1 below, and the holding time of the second conducting current value ( T ₂ ) was applied at 100-500 ms:

[Equation 1]

T ₂ ≤ T ₁ ≤ 3*T ₂ (unit: ms).

Comparative Example 1

Resistance spot welding was performed in the same manner as in the above embodiment, except that the current was applied by applying a current value of 6 to 8.5 kA as shown in FIG. 3(a).

Comparative Example 2

When the first power is applied, a current value of 7 to 8 kA as shown in Fig. 3(b) is applied to apply a spike type power pattern (high current, short time), and then a current value of 4.5 to 6 kA is applied. Resistance spot welding was performed in the same manner as in the above embodiment, except that the second current was applied by applying it.

FIG. 7(a) shows the primary energization effect of Comparative Example 2 (spike type), that is, the nugget formation and corona bond formation behavior formed immediately before the application of the secondary energization, and FIG. After the first energization of the embodiment, the nugget formation and corona bond formation behavior at the time when the current is applied to the second energization current value with a slope is shown, and the current value according to the present invention has a slope and the effect that appears when applied is compared. I did it. _{That is, when comparing the behavior formed immediately before the application of the second current value (I 2} , T _{2 in} FIG. 2) when the second current is applied as shown in FIG. 7, the present invention is compared to the conventional spike-type primary current, corona It can be seen that the bond width is enlarged and has an excellent effect of suppressing the occurrence of flying.

Figure 8 (a) is a comparative example 1 shows the welding nuggets, Figure 8 (b) is a photograph showing the example welding nuggets. Referring to FIG. 8, when applying the resistance spot welding according to the embodiment of the present invention, it can be seen that the nugget size was increased more than the nugget size of Comparative Example 1.

9 is a comparison of the fracture phenomenon occurring in the welding portion of Comparative Example 2. Referring to FIG. 9, in Comparative Example 2, it can be seen that when a spike type energization pattern is applied, the fusion portion nugget is fractured.

10 is a graph showing the size of the nugget diameter according to the current range applied to the secondary energization during resistance spot welding of Examples and Comparative Example 2 of the present invention. Referring to FIG. 10, it can be seen that in the embodiment of the present invention, compared to Comparative Example 2, the proper welding current range is significantly expanded, and even when a high current is applied, breakage of the nugget or welding defect does not occur. On the other hand, in the case of Comparative Example 2, when applied with a current of about 6.7 kA or more, it can be seen that the nugget size no longer grows and fractures, etc., resulting in welding defects.

Since more energy is applied as the value of the conventionally applied current increases, the size of the nugget increases, and thus the breaking strength increases, and when it is enlarged beyond a certain size, the shape of the break is also excellently improved from the breakage of the nugget to the break of the HAZ part.

However, with the conventional method, in the high current condition of the maximum level of 6.7kA or higher, the welding is impossible, and the nugget size was limited to the maximum level of 6.25mm.

On the other hand, the present invention is capable of welding up to 9.5kA up to a high current, so the nugget size has grown to a level of 8.25mm compared to the prior art.As a result, when the current condition range exceeding the nugget size of the minimum required quality is based on 5.92mm, the prior art On the other hand, the developed technology was able to significantly expand the range of available welding current, not only satisfying 5.92mm in all current conditions within the range of 6.0 to 9.5kA, but also secured excellent nugget quality of up to 8.25mm level.

Simple modifications or changes of the present invention can be easily implemented by those of ordinary skill in the art, and all such modifications or changes can be considered to be included in the scope of the present invention.

Claims (7)

Contacting and pressing a pair of electrodes on the upper and lower portions of the portion to be welded of the steel sheet to be joined;
Applying and maintaining a preliminary current value (Ip) to the portion to be welded while maintaining the pressure of the electrode to perform preliminary current;
_{Applying and maintaining a first current value (I 1} ) to the pre-energized welding target portion while maintaining the pressurization to perform primary energization; And
While maintaining the pressurization, performing secondary energization by increasing and maintaining a current with a slope _{from the first} current value (I 1) to the second current value (I _{2 ), and includes,}
The preliminary conduction current value Ip is a current value equal to or greater than _{the first conduction current value I 1,}
Wherein the holding time of the first switching current value (T ₂₎ and the holding time of the second switching current value (T ₂₎ is to satisfy a relationship of the formula 1,
Resistance spot welding method of a steel sheet, characterized in that when the current increases from the first current value to the second current value, the second current value is increased for a period of 50% or less of the holding time (T2):
[Equation 1]
T ₂ ≤ T ₁ ≤ 3*T ₂ (unit: ms).
The method of claim 1,
The bonded steel sheet is a resistance spot welding method of a steel sheet, characterized in that it comprises at least one of ultra-high strength steel and hot forming steel sheet having a tensile strength of 540 MPa or more.
delete The method of claim 1,
The first conductive current value (I ₁ ) is a resistance spot welding method of a steel sheet, characterized in that it satisfies the relationship of Equation 2 below:
[Equation 2]
3.0kA ≤ I ₁ ≤ Maximum current (kA) of non-flying of the bonded steel sheet.
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